The present invention relates to degradable balls, methods for their manufacture and methods for use in temporarily sealing perforations in subterranean well bore applications. In particular, at least in some embodiments, the present invention relates to perforation balls that comprise carboxylic acids, fatty alcohols, fatty acid salts, or esters.
“Perforation balls” are substantially spherical balls that may be used to substantially plug perforations during a hydraulic fracturing or acidizing stimulation treatment, or for any other fluid injection treatment, typically for the purpose of diverting flow of the treatment fluid (e.g., the fracturing fluid or the acidic treatment fluid) to other zones of interest within the formation. Most commercially available ball sealers are either a solid material or will have a solid, rigid core which resists extrusion into or through a perforation in the formation and an outer covering sufficiently compliant to seal, or significantly seal, the perforation. The ball sealers should not be able to penetrate the perforation since penetration would block flow through the perforation and could result in permanent damage to the flow characteristics of the well. Commercially available ball sealers are typically spherical with a hard, solid core made from nylon, phenolic, syntactic foam, or aluminum. The solid cores may be covered with rubber to protect them from solvents and to enhance their sealing capabilities.
It is common practice in completing oil and gas wells to set a string of pipe, known as casing, in the well and use a cement sheath around the outside of the casing to isolate the various formations penetrated by the well. To establish fluid communication between the hydrocarbon-bearing formations and the interior of the casing, the casing and cement sheath are perforated, typically using a perforating gun or similar apparatus. At various times during the life of the well, it may be desirable to increase the production rate of hydrocarbons using appropriate treatment fluids such as acids, water-treatment fluids, solvents or surfactants. If only a short, single pay zone in the well has been perforated, the treating fluid will flow into the pay zone where it is needed. As the length of the perforated pay zone or the number of perforated pay zones increases, the placement of the treating or stimulation fluid in the regions of the pay zones where it is needed becomes more difficult. For instance, the strata having the highest permeability will most likely consume the major portion of a given stimulation treatment, leaving the least permeable strata virtually untreated.
Generally, the ball sealers are pumped into the well bore along with the formation treating fluid and are carried down the well bore and onto the perforations by the flow of the fluid through the perforations into the formation. The balls seat upon the opening to the perforations receiving the majority of fluid flow and, once seated, are held there by the pressure differential across the perforations. The ball sealers are injected at the surface and transported downhole by a treating fluid. Other than a ball injector and possibly a ball catcher, no special or additional treating equipment is required. Some of the advantages of utilizing ball sealers as a diverting agent include ease of use, positive shutoff, no involvement with the formation, and low risk of incurring damage to the well. Ball sealers are typically designed to be chemically inert in the environment to which they are exposed; to effectively seal, yet not extrude into the perforations; and to release from the perforations when the pressure differential into the formation is relieved.
Most perforation balls are made with materials that are stable under downhole conditions, and thus, following a treatment, need to be recovered from the well bore or otherwise removed from the treatment interval. Perforation balls which have a density greater than that of the wellbore fluid, called “sinkers”, may be flowed off the perforation openings and allowed to fall into the bottom of the wellbore. This may be undesirable because the accumulation of balls in the bottom of the well may hamper or prevent future production or service work on the well. Balls which have a density less than that of the wellbore fluid, or “floaters”, may be flowed back to the surface and captured for possible reuse. This clean-up activity may be undesirable as it can delay further operations at the well and adds complications to the well treatment process. It is desirable to avoid either of these processes, and it is desirable that the perforation balls degrade downhole in such a manner as to not form undesirable products that may negatively impact any operations. More particular, it is desirable that such balls degrade in a predictable manner, typically within a few hours or days.
Commercially available degradable perforation balls are not satisfactory, inter alia, because of their limited temperature range usability. For lower temperature ranges, these are usually made from polyvinyl alcohol (“PVA”) and/or polyvinyl acetate (“PVAC”). For higher temperature applications, balls may be made from blends of polyethylene oxide (“PEO”), poly(propylene oxide) (“PPO”), and polylactic acid (“PLA”). Perforation balls made from these materials may soften and become ineffective or transform into an undesirable material in certain conditions. As a result, they may become ineffective as perforation sealers. Further, under these conditions, the polymeric residue of these perforation balls may be forced into the perforation tunnels, plugging them and reducing conductivity of the formation. This is undesirable.